During the past year we have been able to purify large quantities of guanylate cyclase from sea urchins. The purified enzyme showing a single band in gel electrophoresis was able to convert 16 micromole/min/mg GTP to cyclic GMP. The enzyme appears to contain 2 subunits of apparent M.W.59000. After repeated injections of purified enzyme in rabbits over a period of one year, the sera contained a specific antibody against guanylate cyclase. In mice, the antibody appears to be produced within 2 months of injection. The antisera showed a very discrete single precipitin line against purified as well as partially purified guanylate cyclase. It also inhibited guanylate cyclase. The production of monoclonal antibody is under progress.